Can blind toads teach us something new about invasive species?

Amphibians are currently experiencing a dramatic man-made extinction crises, with a higher percentage of endangered species than any other vertebrate groups including mammals, fish and birds. However, at the same time, some amphibians introduced by man into new territories have become phenomenally abundant and invasive, so much so that several of them are rated among the worst 100 invasive species on the planet.

Invasive species are one of the main drivers worldwide of biodiversity loss and also a major source of economic damage and as such understanding their ecology, mechanisms for invasiveness, impacts and ways to mitigate their effects are all paramount. This is especially relevant given that the ongoing and fast globalisation means that future introductions, both accidental and deliberate -mostly illegal using animals from the pet industry- are very likely to further increase.

The most famous example of an invasive amphibian is the cane toad (Rhinella marina), an imposing Central and South-American species which has been introduced into Australia in the 1930s in a misguided attempt to control a beetle in sugarcane plantations. The cane toad has now spread to most parts of north-western Australia and is causing untold damage to entire ecosystems as naïve Australian species, from monitor lizards and snakes to birds and mammals, eat the toads and die from poisoning given that it can produce large quantities of toxic secretions from its skin as a defensive mechanism. The scale of the invasion as well as the pace of the spread, currently estimated at around 40-60km per year, illustrates the remarkable potential of such species to adapt and take over a new environment following an introduction.

In another recent and fascinating example of invasive potential and adaptability of some amphibians Tolledo and Toledo (2015) report on a related species, the Cururu toad (Rhinella jimi), also from South America, which has been introduced in an effort to control the insect population, to the Fernando de Noronha archipelago, off the west coast of Brazil. The introduction was not official or state-sanctioned and therefore the information about it is scarce but most likely it took place around 100 years ago. Since then, the toad population has hugely expanded and become invasive but is suffering from a very high degree of malformations, both in tadpoles and in adults, including a very large proportion of one-eyed and even blind individuals. Despite this, both half-blind and fully blind individuals were able to survive on the island in similar numbers to normal individuals.

The fact that such individuals have adapted to cope with such serious deformities and survive to hunt without relying on vision shows remarkable plasticity and adaptability, all necessary traits for invasive species. The paper describes captive experiments with both blind, half-blind and normal toads from the island where the two groups exhibited completely different foraging strategies, with the blind toads entirely switching their foraging mechanism from active hunting at night relying on vision, to sit and wait predator, relying exclusively on tactile cues and waiting for prey items to come to them and touch them. Equally, the diet composition varied significantly between the groups, with more fast-moving prey items and overall larger numbers of prey for the toads with normal eyesight.

The cause for the huge range of malformations of the Cururu toads in this island remains unknown but this represents a possibly unique experiment showing cascading effects on the toad population following introduction but also remarkable adaptability within a very short period of time.

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